Solid electrolyte and lithium ion battery

ABSTRACT

A solid electrolyte capable of securing grain boundary resistance even when firing is performed at a relatively low temperature and a battery using the solid electrolyte are provided. The solid electrolyte includes a first electrolyte which contains a lithium composite metal compound, and a second electrolyte which contains Li and at least two kinds of metal elements selected from group 5 elements in period 5 or higher or group 15 elements in period 5 or higher.

TECHNICAL FIELD

The present invention relates to a solid electrolyte and a lithium ionbattery using the solid electrolyte.

BACKGROUND ART

As a solid electrolyte for lithium ion battery, for example, PTL 1discloses a silicon-containing lithium lanthanum titanate compositesolid electrolyte material and a method of manufacturing the same.According to PTL 1, amorphous silicon (Si) or a Si compound isintroduced to a grain boundary between lithium lanthanum titanatecrystal particles by adding a lithium lanthanum titanate complexcompound to a silicon precursor solution, heating and drying the mixturethereof, and then pelleting and firing the mixture. This indicates thatthe grain boundary conductivity is markedly improved.

In addition, for example, PTL 2 discloses a method of forming a titaniumoxide type solid electrolyte by heating and firing a precursor solutioncontaining water and a water-soluble titanium compound. According to PTL2, since the precursor solution can contain each element supply sourcesuch as titanium, lithium (Li), and lanthanum (La) at a relatively highconcentration, titanium oxide type solid electrolyte having a desiredvolume can be formed by reducing the number of times of applying andfiring of a solution as compared with a sol-gel method using alcohol asa solvent.

CITATION LIST Patent Literature

PTL 1: JP-A-2011-529243

PTL 2: JP-A-2003-346895

SUMMARY OF INVENTION Technical Problem

However, in the above PTLs 1 and 2, when firing is performed at a hightemperature of 1000° C. or higher, there is a concern in that Li isextracted from a fired body or by-products are generated by a heattreatment, and thereby a composition of the fired body is changed. Here,when a temperature of the heat treatment is set to be lowered in orderto suppress the composition change of the fired body, there is a problemin that an interface between crystal particles is not sufficientlyfired, and thereby grain boundary resistance is increased.

In addition, in a case where the solid electrolyte is formed using a wetchemistry method disclosed in the above PTLs 1 and 2, there is also aproblem in that a product forms a uniform layer, and thus an interfacein the product is clear and interface resistance is likely to occur.

Solution to Problem

The invention has been made to solve at least a part of the aboveproblems, and can be realized as the following aspects or applicationexamples.

[Application Example]

A solid electrolyte according to this application example includes afirst electrolyte which is garnet-type or garnet-like type crystallineand a second electrolyte which is ion-conductive amorphous, in which thefirst electrolyte is a lithium composite metal compound, and the secondelectrolyte contains Li and at least two kinds of metal elementsselected from group 5 elements in period 5 or higher or group 15elements in period 5 or higher.

According to this application example, from the viewpoint of aconfiguration in which the first electrolyte which is crystalline andthe second electrolyte which is amorphous are bonded to each other, aneffect of reducing resistance occurring at a crystal interface can beobtained as compared with a case where the first electrolytes which arecrystalline are directly bonded to each other. In addition, the aboveconfiguration contributes to stabilization of a cubic crystal in firingof the garnet-type or garnet-like type crystalline at a low temperature,and thus it is possible to secure ionic conductivity in crystallinewithout performing the firing at the high temperature. In addition, aportion of metal elements contained in the lithium composite metalcompounds constituting the first crystalline electrolyte is substitutedwith at least two kinds of metal elements, contained in the secondelectrolyte, and selected from group 5 elements in period 5 or higher orgroup 15 elements in period 5 or higher, and thereby a concentrationgradient of the metal element is generated between the first crystallineelectrolyte and the second amorphous electrolyte. With this, a boundarybetween the first electrolyte and the second electrolyte becomesunclear, and as compared with a case where the boundary is clear, it ispossible to reduce the grain boundary resistance and realize high ionicconductivity.

In the solid electrolyte according to the application example, it ispreferable that an atomic crystal radius of the metal element is 78 pmor more.

According to this configuration, even when the heat treatment isperformed, the metal element having an atomic crystal radius of 78 pm ormore is hardly extracted from the first electrolyte which is garnet-typeor garnet-like type crystalline, and thus stable ionic conductivity canbe obtained.

In the solid electrolyte according to the application example, it ispreferable that the first electrolyte is a lithium composite metalcompound containing Li, La, and Zr, and two kinds of the metal elementsare selected from Nb, Ta, Sb, and Bi.

According to this configuration, a portion of Zr among the lithiumcomposite metal compounds constituting the first electrolyte issubstituted with two kinds of the metal elements selected from Nb, Ta,Sb, and Bi, and thus excellent ionic conductivity can be realized.

In the solid electrolyte according to the application example, the firstelectrolyte is a lithium composite metal compound containing Li, La, andZr, and three kinds of the metal elements may be selected from Nb, Ta,Sb, and Bi.

According to this configuration, a portion of Zr among the lithiumcomposite metal compounds constituting the first electrolyte issubstituted with three kinds of the metal elements selected from Nb, Ta,Sb, and Bi, and thus a solid electrolyte having excellent ionicconductivity can be realized.

The solid electrolyte according to the application example preferablyfurther includes a third electrolyte which is amorphous and is formed ofan oxide containing Li and B.

According to this configuration, since the first electrolyte is bondedto not only the second electrolyte but also the third electrolyte, asolid electrolyte having high ionic conductivity can be realized byeffectively utilizing the first electrolyte.

[Application Example]

A lithium ion battery according to this application example includes asolid electrolyte layer formed of the solid electrolyte according to theabove application example; an electrode provided on one surface of thesolid electrolyte layer; and a current collector provided on the othersurface of the solid electrolyte layer.

According to this application example, a solid electrolyte in which thegrain boundary resistance is reduced and the ionic conductivity issecured is used, and thus it is possible to provide a lithium ionbattery having excellent charge and discharge characteristics.

In the lithium ion battery according to the application example, it ispreferable that the electrode is formed of metallic lithium, and apositive electrode active material layer containing Li is providedbetween the other surface of the solid electrolyte layer and the currentcollector.

According to this configuration, an electrode and a positive electrodeactive material layer which serve as lithium sources are provided, andthus it is possible to provide a lithium ion battery having excellentcharge and discharge characteristics and a large capacity.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view illustrating a configuration of alithium ion battery.

FIG. 2 is a schematic view illustrating the configuration of a solidelectrolyte layer of a first embodiment.

FIG. 3 is a flowchart illustrating a method of manufacturing the solidelectrolyte of the first embodiment.

FIG. 4 is a schematic diagram illustrating a configuration of a solidelectrolyte layer of a second embodiment.

FIG. 5 is a flowchart illustrating a method of manufacturing the solidelectrolyte of the second embodiment.

FIG. 6 is a schematic sectional view illustrating the method ofmanufacturing the solid electrolyte of the second embodiment.

FIG. 7 is a graph illustrating measurement results of X-ray diffractionintensity of Examples 1 to 5.

FIG. 8 is a graph illustrating measurement results of X-ray diffractionintensity of Comparative Examples 1 and 2.

FIG. 9 is a table illustrating analysis results of X-ray diffraction ofa solid electrolyte of Examples 1 to 5 and Comparative Examples 1 and 2.

FIG. 10 is a table indicating a measurement result and a bulk density oflithium ionic conductivity in the solid electrolyte pellet of Examples 1to 5, and Comparative Examples 1 and 2.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments embodying the invention will be described withreference to the drawings. Note that, the drawing to be used isappropriately displayed by being enlarged or reduced so as to makeexplanation parts recognizable.

First Embodiment

First, an example of a lithium ion battery to which a solid electrolyteof this embodiment is applied will be described with reference toFIG. 1. FIG. 1 is a schematic sectional view illustrating aconfiguration of the lithium ion battery.

As illustrated in FIG. 1, the lithium ion battery 10 has a configurationin which a current collector 1, an active material layer 2, a solidelectrolyte layer 3, and an electrode 4 are stacked in this order. Thelithium ion battery 10 is in a disk form having an outer shape of, forexample, φ 3 mm to 30 mm and a thickness of 150 μm to 200 μm(micrometer). Such a thin lithium ion battery 10 may be used alone, or aplurality of lithium ion batteries 10 may be stacked to be used.Hereinafter, each configuration of the lithium ion battery 10 will bedescribed.

Examples of the current collector 1 include one kind of metal (metalsimple substance) selected from metal groups of copper (Cu), magnesium(Mg), titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn),aluminum (Al), germanium (Ge), indium (In) gold (Au), platinum (Pt),silver (Ag), and palladium (Pd), and an alloy formed of two or morekinds of metals selected from the metal groups.

The shape of the current collector 1 maybe a plate shape, a foil shape,a net shape, or the like, and the surface thereof may be smooth, orirregularities may be formed thereon. A thickness of such a currentcollector 1 is, for example, approximately 20 μm.

In the lithium ion battery 10, a material of the active material layer 2is differentiated depending on whether the current collector 1 is usedon the positive electrode side or on the negative electrode side.

In a case where the current collector 1 is used on the positiveelectrode side, the active material layer 2 is formed by using apositive electrode active material. Examples of the positive electrodeactive material include a lithium composite metal compound including,for example, two or more kinds of metal elements containing lithium(Li). More specifically, examples of the lithium composite metalcompound include a lithium composite oxide such as LiCoO₂, LiNiO₂,LiMn₂O₄, Li₂Mn₂O₃, LiFePO₄, Li₂FeP₂O₇, LiMnPO₄, LiFeBO₃, Li₃V₂ (PO₄)₃,Li₂CuO₂, Li₂FeSiO₄, and Li₂MnSiO₄. In addition, other than the lithiumcomposite oxide, lithium composite fluoride such as LiFeF₃ may be used.Further, those in which a part of atoms of these lithium composite metalcompounds is substituted with other transition metals, typical metals,alkali metals, alkali rare earths, lanthanoids, chalcogenides, halogensand the like are also included. In addition, a solid solution of theselithium composite metal compounds may be used as the positive electrodeactive material.

In a case where the current collector 1 is used on the negativeelectrode side, the active material layer 2 is formed by using anegative electrode active material. Examples of the negative electrodeactive material include a silicon-manganese alloy (Si—Mn), asilicon-cobalt alloy (Si—Co), silicon-nickel alloy (Si—Ni), niobiumpentoxide (Nb₂O₅), vanadium pentoxide (V₂O₅), titanium oxide (TiO₂),indium oxide (In₂O₃), zinc oxide (ZnO), tin oxide (SnO₂), nickel oxide(NiO), indium oxide (ITO) doped with tin (Sn), zinc oxide (AZO) dopedwith aluminum (Al), zinc oxide (GZO) doped with gallium, tin oxide (ATO)doped with antimony, tin oxide (FTO) doped with fluorine (F), carbonmaterial, a material in which lithium ions are inserted between layersof a carbon materials, anatase phase of TiO₂, a lithium composite oxidesuch as Li₄Ti₅O₁₂ and Li₂Ti₃O₇, and metallic lithium.

A thickness of such an active material layer 2 is, for example,approximately 20 μm to 400 μm.

In the case where the current collector 1 is used on the positiveelectrode side, an electrode 4 becomes negative electrode. In this case,as the current collector 1, for example, aluminum (Al) can be used, andas the electrode 4, for example, metallic lithium can be used. Athickness of the electrode 4 is, for example, approximately 40 μm. Ifmetallic lithium is used as the electrode 4 which functions as anegative electrode, it becomes a lithium source in charge and discharge,and thereby a large capacity lithium ion battery 10 can be realized.

The solid electrolyte layer 3 to which the solid electrolyte of theembodiment is applied includes a first electrolyte which is crystalline,and a second electrolyte which is ion-conductive amorphous, and hasexcellent ionic conductivity. A thickness of the solid electrolyte layer3 is, for example, 50 nm (nanometer) to 100 μm. The lithium ion battery10 provided with such a solid electrolyte layer 3 has excellent batterycharacteristics (charge and discharge characteristics). Hereinafter, thesolid electrolyte layer 3 in the lithium ion battery 10 of theembodiment will be described in detail.

Although described in detail later, the solid electrolyte layer 3 may becombined with the active material layer 2.

<Solid Electrolyte Layer>

FIG. 2 is a schematic view illustrating a configuration of the solidelectrolyte layer of the first embodiment. As illustrated in FIG. 2, thesolid electrolyte layer 3 includes at least a first part 3A containing afirst electrolyte 31 which is crystalline, a second part 3B containing asecond electrolyte 32 which is ion-conductive amorphous, and a thirdpart 3C which is a void. The void communicates with the inside of thesolid electrolyte layer 3. That is, the solid electrolyte layer 3 isporous.

A structure of such a solid electrolyte layer 3 can be confirmed by, forexample, a transmission electron microscope, but a boundary between thefirst part 3A and the second part 3B in the solid electrolyte layer 3 ofthe embodiment is not necessarily clear, and specifically, as describedlater, the concentration of at least two kinds of the metal elementscontained in the first electrolyte 31 and the second electrolyte 32 iscontinuously changed between the first part 3A and the second part 3B,and thereby the boundary between the first part 3A and the second part3B becomes unclear. Note that, as to the configuration of the solidelectrolyte layer as illustrated in FIG. 2, a state of observation isschematically illustrated by the transmission electron microscope anddoes not necessarily correspond to a state of actual observation.

As the electrolyte material constituting the first electrolyte 31 andthe second electrolyte 32, oxide, sulfide, nitride, hydride whichcontain lithium such as Li_(5.5)Ga_(0.5)La₃Zr₂O₁₂,Li_(3.4)V_(0.6)Si_(0.4)O₄, Li₁₄ZnGe₄O₁₆, Li_(3.6)V_(0.4)Ge_(0.6)O₄,Li_(1. 3)Ti_(1.7)A_(10.3)(PO₄)₃, Li_(2.88)PO_(3.73)N_(0.14r), LiNbO₃,Li_(0.35)La_(0.55)TiO₃, Li₃NI₂, Li₆NBr₃, Li₂SO₄, Li₄SiO₄, Li₄GeO₄,Li₃VO₄, Li₄GeO₄—Zn₂GeO₂, LiMoO₄, Li₃PO, Li₄ZrO₄, Li_(2+x)Cl_(−x)B_(x)O₃,LiBH₄, Li_(7−x)Ps_(6−x)Cl_(x), and Li₁₀GeP₂S₁₂ or crystalline, amorphousand partially crystallized glasses of these partially substitutedsubstances can be suitably used.

In the embodiment, it is preferable that the first electrolyte 31exhibits excellent ionic conductivity and is electrochemically stable,and the garnet-type or garnet-like type crystalline represented byindicative formula of Li_(7−x)A_(2−x)M1_(x)M2O₁₂ (0.1≤x≤0.6) is used.

In the indicative formula, as A, any metal element forming thegarnet-type or garnet-like type crystal can be used. Particularly, inorder to form a crystal with high ionic conductivity, zirconium (Zr) ispreferable.

M1 is the metal element of the invention, and any element forming thegarnet-type or garnet-like type crystal can be used. Particularly, inorder to form a crystal with high ionic conductivity, at least two kindsof metal elements are preferably selected from niobium (Nb) and tantalum(Ta) of the group 5 elements in the period 5 or higher, and antimony(Sb) and bismuth (Bi) of the group 15 elements in the same period 5 orhigher.

As for atomic crystal radii (unit: pm (picometer)) of these metalelements, Nb is 78 pm, Ta is 78 pm, Sb is 90 pm, and Bi is 117 pm, whichare all 78 pm or more (Crystal·molecular structure design program,CrystalMaker (registered trademark), manufactured by Hulinks).

As M2, any metal element forming the garnet-type or garnet-like typecrystal can be used. Particularly, a lanthanide element is preferable,and lanthanum (La) is particularly preferable in order to form a crystalwith high ionic conductivity.

The second electrolyte 32 is amorphous and formed of the above-describedelectrolyte materials, and similar to the first electrolyte 31, containsat least two kinds of the metal elements selected from Nb and Ta of thegroup 5 elements in the period 5 or higher, and Sb and Bi of the group15 elements in the same period 5 or higher.

<Method of Manufacturing Solid Electrolyte>

Next, a method of manufacturing the solid electrolyte for constitutingthe solid electrolyte layer 3 will be described with reference to FIG.3. FIG. 3 is a flowchart illustrating a method of manufacturing thesolid electrolyte of the first embodiment. The method of manufacturingthe solid electrolyte of the embodiment is a wet method, and includes ametal compound solution preparing step (step S1), a solid electrolyteprecursor solution preparing step (step S2), a solid electrolytesynthesizing step (step S3), and a product firing step(step S4).

In a metal compound solution preparing step of step S1, for each metalcontained in the solid electrolyte, each metal compound is obtained as ametal compound, and a metal compound solution is prepared by dissolvingthe metal compound in a solvent. Examples of the metal compound preparedin the embodiment include a lithium compound, a lanthanum compound, azirconium compound, a niobium compound, a tantalum compound, an antimonycompound, and a bismuth compound.

Examples of the lithium compound (lithium source) include lithium metalsalts such as lithium chloride, lithium nitrate, lithium acetate,lithium hydroxide, and lithium carbonate, lithium alkoxide such aslithium methoxide, lithium ethoxide, lithium propoxide, lithiumisopropoxide, lithium butoxide, lithium isobutoxide, lithium secondarybutoxide, lithium tertiary butoxide, and dipivaloyl methanol tritium,and these can be used alone or two or more kinds thereof can be used incombination.

Examples of the lanthanum compound (lanthanum source) include lanthanummetal salt such as lanthanum chloride, lanthanum nitrate, lanthanumacetate, and lanthanum alkoxide such as lanthanum methoxide, lanthanumethoxide, lanthanum propoxide, lanthanum isopropoxide, lanthanumbutoxide, lanthanum isobutoxide, lanthanum secondary butoxide, lanthanumtertiary butoxide, and dipivaloyl methanate lanthanum, and these can beused alone or two or more kinds thereof can be used in combination.

Examples of the zirconium compound (zirconium source) include zirconiummetal salt such as zirconium chloride, zirconium oxychloride, zirconiumoxynitrate, zirconium oxyacetate, and zirconium acetate, zirconiumalkoxide such as zirconium methoxide, zirconium ethoxide, zirconiumpropoxide, zirconium isopropoxide, zirconium butoxide, zirconiumisobutoxide, zirconium secondary butoxide, zirconium tertiary butoxide,and dipivaloyl methanatozirconium, and these can be used alone or two ormore kinds thereof can be used in combination.

Further, examples of the niobium compound (niobium source) as a compoundwhich is a metal element among the group 5 elements in period 5 orhigher, and having an atomic crystal radius of 78 pm or more include aniobium metal salt such as niobium chloride, niobium oxychloride,niobium oxalate, and niobium acetylacetone, niobium alkoxide such asniobium ethoxide, niobium propoxide, niobium isopropoxide, and niobiumsecondary butoxide, and these can be used alone or two or more kindsthereof can be used in combination.

Further, examples of the tantalum compound (tantalum source) as acompound which is a metal element among the group 5 elements in period 5or higher, and having an atomic crystal radius of 78 pm or more includetantalum metal salts such as tantalum chloride, and tantalum bromide,tantalum alkoxide such as pentamethoxytantalum, pentaethoxytantalum,pentaisopropoxytantalum, penta normal propoxy tantalum,pentaisobutoxytantalum, penta normal butoxy tantalum, penta secondarybutoxy tantalum, and penta tertiary butoxy tantalum, and these can beused alone or two or more kinds thereof can be used in combination.

In addition, examples of the antimony compound (antimony source) as acompound which is a metal element among the group 15 elements in period5 or higher, and having an atomic crystal radius of 78 pm or moreinclude antimony metal salt such as antimony bromide, antimony chloride,and antimony fluoride, antimony alkoxide such as trimethoxyantimony,triethoxyantimony, triisopropoxyantimony, trinormal propoxy antimony,triisobutoxy antimony, and trinormal butoxy antimony, and these can beused alone or two or more kinds thereof can be used in combination.

Similarly, examples of the bismuth compound (bismuth source) as acompound which is a metal element among the group 15 elements of period5 or higher, and having an atomic crystal radius of 78 pm or moreinclude bismuth metal salt such as bismuth tribromide, bismuthtrichloride, bismuth trifluoride, bismuth triiodide, bismuth nitrate,bismuth oxychloride, bismuth tribenzoate, bismuth citrate, bismuthacetate, and bismuth 2 ethylhexanoate, bismuth alkoxide such as bismuthtriethoxide, bismuth tri-normal propoxide, bismuth triisopropoxide,bismuth trinormal butoxide, bismuth triisobutoxide, bismuth canedarybutoxide, vismaster sarybutoxide, and bismuth tritertary amyloxide, andthese can be used alone or two or more kinds thereof can be used incombination.

As the solvent, a single solvent or a mixed solvent of water and anorganic solvent capable of dissolving each of a lithium compound, alanthanum compound, a zirconium compound, and a metal compound having anatomic crystal radius of 78 pm or more is used.

Such an organic solvent is not particularly limited, and examplesthereof include alcohols such as methanol, ethanol, n-propyl alcohol,isopropyl alcohol, allyl alcohol, and 2-n-butoxyethanol, glycols such asethylene glycol, propylene glycol, butylene glycol, hexylene glycol,pentane diol, hexane diol, heptane diol, and dipropylene glycol, ketonessuch as acetone, methyl ethyl ketone, methyl propyl ketone, and methylisobutyl ketone, esters such as methyl formate, ethyl formate, methylacetate, and methyl acetoacetate, ethers such as diethylene glycolmonomethyl ether, diethylene glycol monoethyl ether, diethylene glycoldimethyl ether, ethylene glycol monomethyl ether, ethylene glycolmonoethyl ether, and dipropylene glycol monomethyl ether, organic acidssuch as formic acid, acetic acid, and propionic acid, and aromaticcompounds such as toluene, o-xylene, and p-xylene.

The metal compound solution is prepared by weighing the above-describedmetal compound so as to have a concentration of molar (mol) unit,introducing the metal compound into a selected solvent, mixing the metalcompound and the solution, and dissolving the mixture. In order todissolve the metal compound sufficiently, if necessary, the solvent iswarmed to be mixed with the metal compound. Then, the process proceedsto step S2.

In the solid electrolyte precursor solution preparing step of step S2,in consideration of the composition of the solid electrolyte to beobtained as a product, a metal compound solution in which the metalcompound is dissolved for each metal source is weighed and then mixedtogether so as to obtain a solid electrolyte precursor solution.Specifically, three kinds of metal compound solutions each containing alithium compound, a lanthanum compound, and a zirconium compound, and aplurality of kinds of metal compound solutions each containing at leasttwo kinds of metal compounds selected from a niobium compound, atantalum compound, an antimony compound, and a bismuth compound aremixed together at a predetermined mixing ratio. Then, the processproceeds to step S3.

In the solid electrolyte synthesizing step of step S3, a solventcomponent is removed by heating the solid electrolyte precursor solutionobtained in the step S2 so as to obtain a product. The removal of thesolvent component is performed in an open state to the atmosphere. Then,the process proceeds to step S4.

In the product firing step of step S4, the product obtained in step S4is fired. Since it is preferable to completely remove the solventcomponent that may remain in the product, the firing step is preferablyperformed stepwise. In the embodiment, a preliminarily fired bodyobtained by temporarily firing the product at a temperature of lowerthan 900° C. is ground by an agate mortar to be finely pulverized. Aprescribed amount of the pulverized preliminarily fired body is weighed,the pulverized preliminarily fired body is put into a die (tabletformer), and is pressure-molded so as to obtain a molded body. Theobtained molded body is placed in a crucible made of magnesium oxide soas not to change the composition at the time of main firing, thecrucible is covered with a lid made of magnesium oxide, and the mainfiring is performed at a temperature of, for example, 900° C. or higherand lower than 1000° C. so as to obtain a solid electrolyte.

According to the method of manufacturing the solid electrolyte of thefirst embodiment, it is possible to obtain a solid electrolyte includinga first electrolyte 31 which is garnet-type or garnet-like typecrystalline, and a second electrolyte 32 which is ion-conductiveamorphous. Since the solid electrolyte (solid electrolyte layer 3) has aconfiguration in which a second part 3B containing the secondelectrolyte 32 is connected to a first part 3A containing the firstelectrolyte 31, a solid electrolyte having high ionic conductivitywithout firing at a high temperature can be realized or manufactured byreducing the resistance occurring at each crystal interface as comparedwith a case of having a configuration in which a plurality ofcrystallines are directly connected.

Further, among the garnet-type or garnet-like type crystals, a portionof Zr is substituted with at least two kinds of metal elements among Nb,Ta, Sb, and Bi which have an atomic crystal radius of 78 pm or more.Since the viewpoint that the metal element is contained also in thesecond electrolyte 32, a concentration of the metal element in the solidelectrolyte is high in the second electrolyte 32 which is amorphous ascompared with that in the first electrolyte 31 which is crystalline.Accordingly, a concentration gradient of the metal element is generatedbetween the first electrolyte 31 and the second electrolyte 32. Withthis, the concentration of the metal element is continuously changed atthe boundary between the first electrolyte 31 and the second electrolyte32, and thus the boundary between the first electrolyte 31 and thesecond electrolyte 32, which can be confirmed by means such as atransmission electron microscope becomes unclear. In other words, ascompared with a case where the boundary between the first electrolyte 31and the second electrolyte 32 is clear, the charge transfer between thefirst electrolyte 31 and the second electrolyte 32 is performedsmoothly, and when using such a solid electrolyte, a solid electrolytelayer 3 having excellent ionic conductivity can be formed.

The lithium ion battery 10 provided with the solid electrolyte layer 3having such an excellent ionic conductivity has excellent batterycharacteristics (charge and discharge characteristics). Further, whenthe current collector 1 is on the positive electrode side, the activematerial layer 2 is formed on the current collector 1 by using thepositive electrode active material, and the electrode 4 is formed as anegative electrode on the solid electrolyte layer 3 using metalliclithium, it is possible to obtain a lithium ion battery having excellentbattery characteristics and a large capacity.

Note that, in the method of manufacturing the solid electrolyte, amongNb, Ta, Sb, and Bi which are the metal elements having an atomic crystalradius of 78 pm or more, any one of Nb, Ta, and Sb is preferablyselected from the viewpoint of exhibiting excellent amorphous formingability of the second electrolyte 32. In addition, a metal elementhaving an atomic crystal radius which is larger than Bi (117 pm) isdifficult to substitute Zr in the garnet-type or garnet-like typecrystal structure, and thus it is preferable to select a metal elementwhich is the same as Bi or has an atomic crystal radius smaller than Bi.

Second Embodiment

Next, the method of manufacturing the solid electrolyte of the secondembodiment will be described with reference to FIGS. 4 to 6. FIG. 4 is aschematic diagram illustrating the configuration of a solid electrolytelayer of the second embodiment, FIG. 5 is a flowchart illustrating amethod of manufacturing the solid electrolyte of the second embodiment,and FIG. 6 is a schematic sectional view illustrating the method ofmanufacturing the solid electrolyte of the second embodiment. The solidelectrolyte of the second embodiment is obtained by adding a thirdelectrolyte to the solid electrolyte of the first embodiment, and thesame reference numerals are given to the same configurations as those ofthe first embodiment, and a detailed description thereof will not berepeated.

As illustrated in FIG. 4, a solid electrolyte layer 3X of the embodimentincludes at least a first part 3A containing a first crystallineelectrolyte 31, a second part 3B containing a second amorphouselectrolyte 32, and a third part 3C containing a third amorphouselectrolyte 33. In other words, the third part 3C which is a void in thesolid electrolyte layer 3 of the first embodiment is filled with thethird electrolyte 33.

For the third electrolyte 33, the above-described electrolyte materialsof the first electrolyte 31 and the second electrolyte 32 can be used,and particularly, a melting point is preferably lower than 900° C.Specifically, a material which has ionic conductivity and is amorphousat room temperature is preferable, and examples thereof include lithiumcomposite oxides containing Li and B such as Li₃BO₃, Li₃BO₃—Li₄SiO₄,Li₃BO₃—Li₃PO₄, Li₃BO₃—Li₂SO₄, and Li₂CO₃—Li₃BO₃.

The ionic conductivity of Li₃BO₃ is approximately 6.0×10⁻¹⁰ S/cm, andthe melting point is approximately 820° C. The ionic conductivity ofLi₃BO₃—Li₄SiO₄ is approximately 4.0×10⁻⁶ S/cm, and the melting point isapproximately 720° C. The ionic conductivity of Li₃BO₃—Li₃PO₄ isapproximately 1.0×10⁻⁷ S/cm, and the melting point is approximately 850°C. The ionic conductivity of Li₃BO₃—Li₂SO₄ is approximately 1.0×10⁻⁶S/cm, and the melting point is approximately 700° C. The ionicconductivity of Li_(2.2)C_(0.8)B_(0.2)O₃ which is Li₂CO₃—Li₃BO₃ type isapproximately 8.0×10⁻⁷ S/cm, and the melting point is 685° C.

As illustrated in FIG. 5, the method manufacturing the solid electrolyteof the embodiment (solid electrolyte layer 3X) includes a thirdelectrolyte adding step (step S5) in addition to the above-describedstep S1 to step S4 of the first embodiment. Hereinafter, step S5 addedto the first embodiment will be described.

In the third electrolyte adding step of step S5, first, powders of thethird electrolyte 33 are prepared. In addition, as illustrated in FIG.6, a predetermined amount of the powders of the third electrolyte 33 isweighed and placed on a fired body 3P after the main firing in step S4.The fired body 3P on which the powders of the third electrolyte 33 areplaced is put into an electric muffle furnace and heated at atemperature which is higher than the melting point of the thirdelectrolyte 33 and is lower than 900° C. which is a temperature at themain firing so as to melt the powders of the third electrolyte 33. Sincethe fired body 3P is a porous body having voids and having a bulkdensity of about 50% to 70%, and the voids communicate internally, amelt of the third electrolyte 33 is impregnated into the voids of thefired body 3P due to capillary phenomenon. Then, by rapidly cooling toroom temperature, the melt is solidified and the solid electrolyte layer3X in which the voids are filled with the third amorphous electrolyte 33is formed.

The bulk density (a ratio of a solid content occupying a total volumeexcluding voids) in the fired body 3P can be obtained by dividing theweight of the fired body 3P by the value obtained by multiplying thevolume of the fired body 3P by the specific gravity. Since the volume ofthe void can be obtained if the bulk density is known, the predeterminedamount of the third electrolyte 33 necessary for filling the void canalso be calculated in advance.

Although it is preferable to fill all of the voids contained in thefired body 3P with the third electrolyte 33, it is not always necessaryto fill all the voids, and some of the voids may remain.

According to the solid electrolyte layer 3X of the second embodiment andthe manufacturing method thereof, when the third part 3C which is thevoid in the first embodiment is filled with the third electrolyte 33,the first crystalline electrolyte 31 can be effectively utilized so asto further improve ionic conductivity. In addition, when the lithium ionbattery 10 is configured to include such a solid electrolyte layer 3X,it is possible to provide the lithium ion battery 10 having moreexcellent battery characteristics (charge and dischargecharacteristics).

Note that, the method of adding the third electrolyte 33 to the firedbody 3P containing the first electrolyte 31 and the second electrolyte32 is not limited to the method of impregnating the melt of the thirdelectrolyte 33. For example, the fired body 3P is again put into theagate mortar, ground, and mixed with the powders of the thirdelectrolyte 33 so as to obtain a mixture. The solid electrolyte layer 3Xwhich is a solid electrolyte molded body may be manufactured in such amanner that the obtained mixture is put into a die (tablet former) andis pressure-molded to form a molded body, and the molded body is firedat a temperature lower than the melting point of the third electrolyte33 and cooled. According to this, since the firing is performed at atemperature lower than the melting point of the third electrolyte 33,even if the heat treatment is performed, Li does not easily come out ofthe molded body.

In addition, in a case where the powders of the third electrolyte 33 aremixed, it is preferable to mix such that the volume ratio of the thirdelectrolyte 33 to the powders of the fired body 3P is about 36% to 75%.When the volume ratio (addition ratio) of the third electrolyte 33 is inthe above range, the particles of the composite of the first electrolyte31 and the second electrolyte 32 in the solid electrolyte are disposedsuch that an average distance therebetween is larger than a mediandiameter of the composite of the first electrolyte 31 and the secondelectrolyte 32, and the third electrolyte is sandwiched between theparticles, thereby easily obtaining high ionic conductivity.

<Method of Manufacturing First Electrolyte>

The method of forming the garnet-type or garnet-like type firstelectrolyte 31 in the solid electrolyte of the first embodiment and thesecond embodiment is not limited to the wet method. Various syntheticmethods can be applied according to desirable application forms ofvarious solution methods such as a solid phase synthesis method, asol-gel method, a metal organic decomposition method (MOD).

Hereinafter, an example of a method of synthesizing garnet-type orgarnet-like type crystal particles by the solid phase synthesis methodwill be described.

In order to obtain garnet-type or garnet-like type crystal particles bythe solid phase synthesis method, at least one kind of compoundcontaining lithium, at least one kind of compound containing zirconium,at least one kind of compound containing lanthanum, and at least twokinds selected from niobium, tantalum, antimony and bismuth are weighedand mixed at a predetermined ratio, and the mixture which is set as astarting material can be synthesized by being heated in an atmospherecontaining at least oxygen gas such as air or an argon-oxygen mixture.

As the raw material containing lithium, for example, oxides such as Li₂Oand Li₂O₂, oxoacid salts such as Li₂CO₃, LiHCO₃, and LiNO₃, hydroxidessuch as LiOH, organic acid salts such as LiCH₃COO, halides such as LiF,an inorganic compound such as Li₃N, a lithium metal, and a lithium alloycan be suitably used in accordance with a desired manufacturing method.If necessary, a plurality of these raw materials containing lithium maybe used in combination. Further, among the constituent elements of thegarnet-type or garnet-like type crystal, double oxides such as Li₂ZrO₃and LiNbO₃ containing at least lithium and containing one or more kindsof elements other than lithium also can be used, and among them, Li₂CO₃is particularly preferable.

As the raw material containing lanthanum, for example, oxides such asLa₂O₃, oxoacid salts such as La₂(CO₃)₃, LaCO₃OH, and La(NO₃)₃,hydroxides such as La(OH)₃, organic acid salts such as La(CH₃COO)₃,halides such as LaF₃, inorganic compounds such as LaC₂, La₂C₃, and LaN,a lanthanum metal, and a lanthanum alloy can be suitably used inaccordance with a desired manufacturing method. If necessary, aplurality of these raw materials containing lanthanum may be used incombination. Further, among the constituent elements of the garnet-typeor garnet-like type crystal, double oxides such as La₂ZrO₇ and La₂Nb₂O₇containing at least lanthanum and containing one or more kinds ofelements other than lanthanum also can be used, and among them, La₂O₃ isparticularly preferable.

As the raw material containing zirconium, for example, oxides such asZrO₂, oxoacid salts such as ZrCO₃, ZrO(NO₃)₂, and ZrOSO₄, hydroxidessuch as ZrO(OH)₂, organic acid salts such as Zr(C₃H₃O₂)₄, halides suchas ZrOCl₂, inorganic compounds such as ZrC, ZrN, a zirconium metal, anda zirconium alloy can be suitably used in accordance with a desiredmanufacturing method. If necessary, a plurality of these raw materialscontaining zirconium may be used in combination. Further, among theconstituent elements of the garnet-type or garnet-like type crystal,double oxides such as Li₂ZrO₃, Li₄ZrO₄, and La₂Zr₂O₇ containing at leastzirconium and containing one or more kinds of elements other thanzirconium also can be used, and among them, ZrO₂ is particularlypreferable.

As a raw material containing niobium, for example, oxides such as Nb₂O₅,oxoacid salts such as Nb₂(CO₃)₅ and Nb₂O₂(SO₄)₃, hydroxides such asNb(OH)₅ and NbO₂OH, halides such as NbCl₅, inorganic compounds such asNbC, NbN, and NbSe₃, a niobium metal, and a niobium alloy can besuitably used in accordance with a desired manufacturing method. Ifnecessary, a plurality of these raw materials containing niobium may beused in combination. Further, among the constituent elements of thegarnet-type or garnet-like type crystal, double oxides such as LiNbO₃and La₂Nb₂O₇ containing at least niobium and containing one or morekinds of elements other than niobium also can be used, and among them,Nb₂O₅ is particularly preferable.

As a raw material containing tantalum, for example, oxides such asTa₂O₅, oxoacid salts such as Ta₂(CO₃)₅, TaO(NO₃)₂, and Ta₂O₂(SO₄)₃,hydroxides such as Ta(OH)₅ and TaO₂OH, halides such as TaCl₅, inorganiccompounds such as TaC, TaN, and TaSe₃, a tantalum metal, and a tantalumalloy can be suitably used in accordance with a desired manufacturingmethod. If necessary, a plurality of these raw materials containingtantalum may be used in combination. Further, among the constituentelements of the garnet-type or garnet-like type crystal, double oxidessuch as LiTaO₃ and La₂Ta₂O₇ containing at least tantalum and containingone or more kinds of elements other than tantalum also can be used, andamong them, Ta₂O₅ is particularly preferable.

As a raw material containing antimony, for example, oxides such asSb₂O₃, oxoacid salts such as Sb₂(CO₃)₃, Sb(HCO₃)₅, and Sb(NO₃)₃,hydroxides such as Sb(OH)₃, halides such as SbCl₅, inorganic compoundssuch as SbC, SbN, and Sb₂Se₃, an antimony metal, an antimony alloy canbe suitably used in accordance with a desired manufacturing method. Ifnecessary, a plurality of these raw materials containing antimony may beused in combination. Further, among the constituent elements of thegarnet-type or garnet-like type crystal, double oxides such as LiSbO₃and La₂Sb₂O₇ containing at least antimony and containing one or morekinds of elements other than antimony also can be used, and among them,Sb₂O₃ is particularly preferable.

As a raw material containing bismuth, for example, oxides such as Bi₂O₃,oxoacid salts such as (BiO)₂CO₃, Bi(CH₃COO)O, BiO(C₆H₄(OH)COO),4BiNO₃(OH)₂.BiO(OH), Bi₂(CO₃)O₂.0.5H₂O, hydroxides such as Bi(OH)₃,organic acid salts such as Bi(C₆H₅C₇ and Bi(C₆H₅COO), halides such asBiF₃ and BiI₃, inorganic compounds such as BiN and BiP, a lithium metal,and a lithium alloy can be suitably used in accordance with a desiredmanufacturing method. If necessary, a plurality of these raw materialscontaining bismuth may be used in combination. Further, among theconstituent elements of the garnet-type or garnet-like type crystal,double oxides such as Bi₂O₃.3ZrO₂ containing at least bismuth andcontaining one or more kinds of elements other than bismuth also can beused, and among them, Bi₂O₃ is particularly preferable.

The powders of the raw materials exemplified above are weighed and mixedso as to satisfy Li_(7 x)La₃Zr_(2 x)Nb_(x)Ta_(x)O₁₂(0<x<0.5), forexample. At this time, there are no particular limits to a particle sizeand a particle size distribution of the powder of the raw material, andthose subjected to sizing so as to equalize the particle size of theparticles or those subjected to a treatment of removing water adsorbedon the surfaces of the particles in advance in a dry atmosphere may beused. A weighing operation may be performed in a dry atmosphere or aninert atmosphere if necessary. Since lithium in the composition formulasometimes desorbs at the time of firing at a high temperature, it maybeadded in an excess amount of about 0.05% to 20% more than thetheoretical composition ratio in advance in accordance with the firingconditions.

As a next step of synthesizing the garnet-type or garnet-like typecrystal by the solid-phase synthesis method, an operation of moldinginto a solid shape so that the particles are in close contact with eachother may be performed in order to promote a solid-phase reaction of theweighed and mixed raw material powder and to improve uniformity. Thereare no particular limits to the shape to be molded and the moldingmethod, and it is possible to use, for example, known methods such as apress process using a die (tablet former) or cold isostatic pressing(CIP). For the purpose of assisting moldability, a so-called binder madeof a polymer may be appropriately added.

A heat treatment for the powder mixed with the raw materials or the rawmaterial powder molded body is performed in a temperature range of 540°C. to 1300° C. depending on the desired fired density and crystallinity.The atmosphere during the heat treatment step is not particularlylimited, but the heat treatment is preferably performed under anatmosphere having air and oxygen mixed at least for a certain period oftime. Further, for the purpose of controlling the desorption reaction ofadditives and elements, it is also possible to perform the treatment inan inert gas atmosphere for a certain period of time or in a fixedtemperature range.

<Method of Manufacturing Lithium Ion Battery>

As a method of manufacturing the lithium ion battery 10 using the solidelectrolyte of the first embodiment or the second embodiment, variousmethods are conceivable. Hereinafter, outlines of various manufacturingmethods will be described.

<Solid Phase Method>

As a material, the active material particle, the particles obtained bypulverizing the fired body of the first electrolyte 31 and the secondelectrolyte 32, and the third electrolyte 33 (if necessary, a conductiveauxiliary agent, a binder, and a solvent may be contained) are prepared.The above materials are mixed using an agate mortar, press-molded (ormolded into a slurry and green sheet), and subjected to a heat treatmentat a temperature lower than the melting point of the third electrolyte33 (after performing a degreasing step in a case of using a binder) soas to manufacture an active material mixture. The active materialmixture in this case is obtained by combining the active material layer2 in the lithium ion battery 10 as illustrated in FIG. 1 with the solidelectrolyte layer 3X of the second embodiment as illustrated in FIG. 4.The current collector 1 and the electrode 4 may be formed in theobtained active material mixture.

<Wet Method>

As a material, the porous body obtained by firing the active materialparticle, the precursor solutions of the first electrolyte 31 and thesecond electrolyte 32, and the precursor solution of the thirdelectrolyte 33 are prepared. The voids in the porous body formed of theactive material particle are filled with the precursor solutions of thefirst electrolyte 31 and the second electrolyte 32 and fired, and thenthe voids in the porous body are filled with the precursor solution ofthe third electrolyte 33 and fired at a temperature of less than themelting point of the third electrolyte 33. That is a method ofcompounding the active material layer 2 and the solid electrolyte layer3X of the second embodiment illustrated in FIG. 4 by applying eachprecursor solution sequentially to the active material layer 2 of thelithium ion battery 10 as illustrated in FIG. 1, followed by drying andfiring the applied precursor solution.

<Using Wet Method and Dry Method in Combination>

As a material, the porous body obtained by firing the active materialparticle, the precursor solutions of the first electrolyte 31 and thesecond electrolyte 32, and the powders of the third electrolyte 33 areprepared. The voids in the porous body formed of the active materialparticle are filled with the precursor solutions of the firstelectrolyte 31 and the second electrolyte 32 and fired, and then a meltobtained by melting the third electrolyte 33 is impregnated andquenched. That is a method compounding the active material layer 2 andthe solid electrolyte layer 3X of the second embodiment illustrated inFIG. 4 by impregnating the melt of the third electrolyte 33 into thefired body obtained by applying each precursor solution to the activematerial layer 2 of the lithium ion battery 10 as illustrated in FIG. 1,followed by drying and firing the applied precursor solution.

In the above-described solid phase method, if the third electrolyte 33is not added, it is possible to obtain an active material mixture inwhich the active material layer 2 and the solid electrolyte layer 3 ofthe first embodiment are compounded. In addition, in the wet method, ifthe third electrolyte 33 as a material is not prepared, it is possibleto compound the active material layer 2 and the solid electrolyte layer3 of the first embodiment.

Next, effects of the solid electrolyte will be specifically describedwith reference to examples and comparative examples.

1. Preparation Example of Metal Compound Solution

<1 mol/kg, Preparing of Butanol Solution of Lithium Nitrate>

1.3789 g of lithium nitrate as a metal compound which is a lithiumsource and 18.6211 g of butanol were weighed in a reagent bottlecontaining a magnetic stir bar and stirred with a magnetic stirrer atroom temperature for 30 minutes, and lithium nitrate was completelydissolved so as to obtain a butanol solution of lithium nitrate at aconcentration of 1 mol/kg.

<1 mol/kg, Preparing of 2-n-butoxyethanol Solution of LanthanumNitrate.6-hydrate>

8.6608 g of lanthanum nitrate as a metal compound which is a lanthanumsource and 11.3392 g of 2-butoxyethanol were weighed in a reagent bottlecontaining a magnetic stir bar and stirred with a magnetic stirrer witha hot plate function at 140° C. for 30 minutes, and lanthanumnitrate.6-hydrate was completely dissolved and gradually cooled to roomtemperature so as to obtain a 2-n-butoxyethanol solution of lanthanumnitrate.6-hydrate at a concentration of 1 mol/kg.

<1 mol/kg, Preparing of Butanol Solution of zirconium-n-butoxide>

3.8368 g of zirconium-n-butoxide as a metal compound which is azirconium source, and 6.1632 g of butanol were weighed in a reagentbottle containing a magnetic stir bar and stirred with a magneticstirrer at room temperature for 30 minutes so as to obtain a butanolsolution of zirconium-n-butoxide at a concentration of 1 mol/kg.

<1 mol/kg, Preparing of 2-n-butoxyethanol Solution of NiobiumPentaethoxide>

3.1821 g of niobium pentaethoxide as a metal compound which is a niobiumsource and 6.8179 g of 2-n-butoxyethanol were weighed in a reagentbottle containing a magnetic stir bar, and stirred with a magneticstirrer at room temperature for 30 minutes so as to obtain a2-butoxyethanol solution of niobium pentaethoxide at a concentration of1 mol/kg.

<1 mol/kg, Preparing of 2-n-butoxyethanol Solution of Antimonytri-n-butoxide>

3.4110 g of antimony tri-n-butoxide as a metal compound which is anantimony source and 6.5890 g of 2-n-butoxyethanol were weighed in areagent bottle containing a magnetic stir bar, and stirred with amagnetic stirrer at room temperature for 30 minutes so as to obtain a2-butoxyethanol solution of antimony tri-n-butoxide at a concentrationof 1 mol/kg.

<1 mol/kg, Preparing of 2-n-butoxyethanol Solution of Tantalumpenta-n-butoxide>

5.4640 g of tantalum penta-n-butoxide as a metal compound which is antantalum source and 4.5360 g of 2-n-butoxyethanol were weighed in areagent bottle containing a magnetic stir bar, and stirred with amagnetic stirrer at room temperature for 30 minutes so as to obtain a2-butoxyethanol solution of tantalum penta-n-butoxide at a concentrationof 1 mol/kg.

2. Preparation Example of Solid Electrolyte Precursor Solution EXAMPLE 1

In Example 1, a Li_(6.75)La₃Zr_(1.75)Nb_(0.25)Ta_(0.25)O₁₂ precursorsolution is prepared as a solid electrolyte precursor solution.

Among the above-described metal compound solutions, 6.7500 g of butanolsolution of lithium nitrate at a concentration of 1 mol/kg, 3.0000 g of2-n-butoxyethanol solution of lanthanum nitrate.6-hydrate at aconcentration of 1 mol/kg, 1.7500 g of butanol solution ofzirconium-n-butoxide at a concentration of 1 mol/kg, 0.2500 g of2-n-butoxyethanol solution of niobium pentaethoxide at a concentrationof 1 mol/kg, and 0.2500 g of 2-n-butoxyethanol solution of tantalumpenta-n-butoxide at a concentration of 1 mol/kg where weighed, andstirred with a magnetic stirrer at room temperature for 30 minutes so asto obtain a Li_(6.75)La₃Zr_(1.75)Nb_(0.2)Ta_(0.255)O₁₂ precursorsolution.

EXAMPLE 2

In Example 2, a Li_(6.75)La₃Zr_(1.75)Nb_(0.25)Sb_(0.25)O₁₂ precursorsolution is prepared as a solid electrolyte precursor solution.

Among the above-described metal compound solutions, 6.7500 g of butanolsolution of lithium nitrate at a concentration of 1 mol/kg, 3.0000 g of2-n-butoxyethanol solution of lanthanum nitrate.6-hydrate at aconcentration of 1 mol/kg, 1.7500 g of butanol solution ofzirconium-n-butoxide at a concentration of 1 mol/kg, 0.2500 g of2-n-butoxyethanol solution of niobium pentaethoxide at a concentrationof 1 mol/kg, and 0.2500 g of 2-n-butoxyethanol solution of antimonytri-n-butoxide at a concentration of 1 mol/kg were weighed, and stirredwith a magnetic stirrer at room temperature for 30 minutes so as toobtain a Li_(6.75)La₃Zr_(1.75)Nb_(0.25)Sb_(0.25)O₁₂ precursor solution.

EXAMPLE 3

In Example 3, a Li_(6.75)La₃Zr_(1.75)Ta_(0.25)Sb_(0.25)O₁₂ precursorsolution is prepared as a solid electrolyte precursor solution.

Among the above-described metal compound solutions, 6.7500 g of butanolsolution of lithium nitrate at a concentration of 1 mol/kg, 3.0000 g of2-n-butoxyethanol solution of lanthanum nitrate.6-hydrate at aconcentration of 1 mol/kg, 1.7500 g of butanol solution ofzirconium-n-butoxide at a concentration of 1 mol/kg, 0.2500 g of2-n-butoxyethanol solution of tantalum penta-n-butoxide at aconcentration of 1 mol/kg, and 0.2500 g of 2-n-butoxyethanol solution ofantimony tri-n-butoxide at a concentration of 1 mol/kg where weighed,and stirred with a magnetic stirrer at room temperature for 30 minutesso as to obtain a Li_(6.75)La₃Zr_(1.75)Ta_(0.25)Sb_(0.25)O₁₂ precursorsolution.

EXAMPLE 4

In Example 4, a Li_(6.75)La₃Zr_(1.75)Nb_(0.25)Ta_(0.25)Sb_(0.25)O₁₂precursor solution is prepared as a solid electrolyte precursorsolution.

Among the above-described metal compound solutions, 6.7500 g of butanolsolution of lithium nitrate at a concentration of 1 mol/kg, 3.0000 g of2-n-butoxyethanol solution of lanthanum nitrate.6-hydrate at aconcentration of 1 mol/kg, 1.7500 g of butanol solution ofzirconium-n-butoxide at a concentration of 1 mol/kg, 0.2500 g of2-n-butoxyethanol solution of niobium pentaethoxide at a concentrationof 1 mol/kg, 0.2500 g of 2-n-butoxyethanol solution of tantalumpenta-n-butoxide at a concentration of 1 mol/kg where weighed, and0.2500 g of 2-n-butoxyethanol solution of antimony tri-n-butoxide at aconcentration of 1 mol/kg where weighed, and stirred with a magneticstirrer at room temperature for 30 minutes so as to obtain aLi_(6.75)La₃Zr_(1.75)Nb_(0.25)Ta_(0.25)Sb_(0.25)O₁₂ precursor solution.

Comparative Example 1

In Comparative Example 1, a Li_(6.75)La₃Zr_(1.75)Nb_(0.25)O₁₂ precursorsolution is prepared as a solid electrolyte precursor solution.

Among the above-described metal compound solutions, 6.7500 g of butanolsolution of lithium nitrate at a concentration of 1 mol/kg, 3.0000 g of2-n-butoxyethanol solution of lanthanum nitrate.6-hydrate at aconcentration of 1 mol/kg, 1.7500 g of butanol solution ofzirconium-n-butoxide at a concentration of 1 mol/kg were weighed, and0.2500 g of 2-n-butoxyethanol solution of niobium pentaethoxide at aconcentration of 1 mol/kg were weighed, and stirred with a magneticstirrer at room temperature for 30 minutes so as to obtain aLi_(6.75)La₃Zr_(1.75)Nb_(0.25)O₁₂ precursor solution. That is, the solidelectrolyte precursor solution of Comparative Example 1 only contains Nbamong four kinds of metal elements (Nb, Ta, Sb, and Bi).

Comparative Example 2

In Comparative Example 2, a Li_(6.75)La₃Zr_(1.75)Nb_(0.25)Si_(0.25)O₁₂precursor solution is prepared as a solid electrolyte precursorsolution.

Among the above-described metal compound solutions, 6.7500 g of butanolsolution of lithium nitrate at a concentration of 1 mol/kg, 3.0000 g of2-n-butoxyethanol solution of lanthanum nitrate.6-hydrate at aconcentration of 1 mol/kg, 1.7500 g of butanol solution ofzirconium-n-butoxide at a concentration of 1 mol/kg, 0.2500 g of2-n-butoxyethanol solution of niobium pentaethoxide at a concentrationof 1 mol/kg, and 0.2500 g of 2-n-butoxyethanol solution of siliconpentaethoxide at a concentration of 1 mol/kg as a metal compound whichis a silicon source were weighed, and stirred with a magnetic stirrer atroom temperature for 30 minutes so as to obtain aLi_(6.75)La₃Zr_(1.75)Nb_(0.25)Si_(0.25)O₁₂ precursor solution. That is,in the solid electrolyte precursor solution of Comparative Example 2, Nbin period 5 among the group 5 elements and Si in period 3 among thegroup 14 elements are contained as a metal element. Note that, an atomiccrystal radius of Si is 40 pm.

3. Synthesizing and firing solid electrolyte

A titanium petri dish having an inner diameter of φ50 mm and a height of20 mm was prepared and each of the solid electrolyte precursor solutionsof Examples 1 to 4 and Comparative Examples 1 and 2 was put into thetitanium petri dish and the titanium petri dish is placed on a hotplate. A temperature of the hot plate is set to be 180° C., and thesolvent is dried for 60 minutes. Subsequently, the temperature of thehot plate is set to be 360° C., and most organic components are firedand decomposed for 30 minutes. Finally, the temperature of the hot platewas set to be 540° C., and preliminary firing was performed to fire anddecompose residual organic components for 60 minutes. The hot plate isslowly cooled to room temperature while the titanium petri dish havingthe obtained preliminarily fired body is placed on the hot plate. Inaddition, each preliminarily fired body is extracted and moved to theagate mortar to be ground and pulverized.

0.2000 g of the pulverized preliminarily fired body is weighed, is putinto a die of φ 10 mm (tablet former), and is subjected to uniaxialpressing at 50 kgN using a handy press machine so as to prepare apreliminarily fired body pellet.

The preliminarily fired body pellet of Examples 1 to 4 and ComparativeExamples 1 and 2 was placed in a crucible made of magnesium oxide andcovered with a lid made of magnesium oxide from above, and then thecrucible is put into an electric muffle furnace to perform firing at900° C. for 12 hours (main firing). After slowly cooling to roomtemperature, the crucible made of magnesium oxide is extracted from theelectric muffle furnace. The solid electrolyte pellet of Examples 1 to4, and Comparative Examples 1 and 2, which is subjected to the mainfiring, is extracted from the crucible made of magnesium oxide.

EXAMPLE 5

In Example 5, the solid electrolyte precursor solution of Example 1 isset as a starting material, and the solid electrolyte pellet which isfired at 900° C. for 12 hours is pulverized in an agate mortar so as toobtain a particulate electrolyte having a median diameter afterpulverization of approximately 6.2 μm. 0.1500 g of the particulateelectrolyte is weighed, and 0.0500 g of Li₃BO₃ (three lithium borate)powder is added thereto. A median diameter of the Li₃BO₃ (three lithiumborate) powder is approximately 6.0 μm. These powders are moved to anagate mortar, 0.2 ml of hexane is added to the agate mortar, and thepowders and added hexane are mixed thoroughly until hexane is completelyvolatilized. The mixed powder is put into a die of φ 10 mm (tabletformer), and is subjected to uniaxial pressing at 50 kgN using a handypress machine so as to make the mixed powder in a pellet form again. Thepellet was placed on a pure gold dish having φ 13 mm, is put into anelectric muffle furnace preheated to 800° C., is subjected to a heattreatment for 10 minutes, and rapidly cooled so as to obtain a solidelectrolyte pellet of Example 5.

4. Evaluation of Solid Electrolyte Pellet of Examples 1 to 5 andComparative Examples 1 and 2

The diameter and thickness of each solid electrolyte pellet after mainfiring (900° C. for 12 hours) were measured with a digital caliper (CD67-S15 PS manufactured by Mitutoyo Corporation.) Also, the weight wasmeasured in unit of 0.1 mg by using analytical balance (ME204Tmanufactured by Mettler Toledo International Inc.) From these values,the bulk density was determined.

In addition, each solid electrolyte pallet was measured by using anX-ray diffractometer (MRD manufactured by Koninklijke Philips N.V.,) soas to confirm a crystal phase.

Further, impedance was measured by the AC impedance method using animpedance measuring device (1260 manufactured by Solartron Metrology),and the bulk, grain boundary, and total lithium ionic conductivity ofthe solid electrolyte which is a product were determined. Specifically,first, gold (Au) having a diameter of φ 8 mm was vapor-deposited on thefront and back surfaces of the pellet of the solid electrolyte bysputtering so as to prepare a deactivated electrode, and the ACimpedance was measured. Subsequently, the AC impedance at the activationelectrode was measured by pressing a lithium metal foil against bothfront and back surfaces of the solid electrolyte pellet on a sputteredgold (Au) electrode.

5. Evaluation Results

FIG. 7 is a graph illustrating measurement results of X-ray diffractionintensity of Example 1 to Example 5, and FIG. 8 is a graph illustratingmeasurement results of X-ray diffraction intensity of ComparativeExamples 1 and 2, FIG. 9 is a table indicating analysis results of thesolid electrolyte of Examples 1 to 5 and Comparative Examples 1 and 2 byX-ray diffraction. Note that, in FIG. 7, reference symbols a to eattached to the graph indicate graphs corresponding to Examples 1 to 5.Similarly, in FIG. 8, reference symbol a attached to the graph indicatesa graph corresponding to Comparative Example 1, and reference symbol bindicates a graph corresponding to Comparative Example 2.

As illustrated in FIGS. 7 and 8, from solid electrolyte of Examples 1 to5 and Comparative Example 1, only diffraction peaks of lithium lanthanumzirconate were observed. In addition, diffraction peaks of lithiumlanthanum zirconate containing metal element selected from among Nb, Taand Sb and doped are not observed. No diffraction peaks related tocontaminating phases indicating generation of contaminant are observed.The doped metal element is considered to be mainly contained in thesecond amorphous electrolyte 32.

On the other hand, as illustrated in FIG. 8, from the solid electrolyteof Comparative Example 2 to which silicon having an atomic crystalradius of 78 pm or less was added, not only the diffraction peak oflithium lanthanum zirconate, but also La₂Zr₂O₇ which indicates thatlithium was removed from the garnet-type crystal structure wereobserved. In addition, a diffraction peak of Li₄SiO₄ which is a compoundwith silicon was observed. It is suggested that Si which is a metalelement of which an atomic crystal radius is less than 78 pm is notsubstituted with the 48 g site in the garnet-type crystal structure andcrystal precipitates at the grain boundary so as to deprive lithium fromthe garnet-type crystal structure, thereby forming double oxide.

That is, as illustrated in FIG. 9, the solid electrolyte of Examples 1to 5 and Comparative Example 1 both contain garnet-type or garnet-liketype lithium lanthanum zirconate, and has no contaminating phase.Although a main phase of the solid electrolyte of Comparative Example 2contains garnet-type or garnet-like type lithium lanthanum zirconate, ithas crystals of lithium-free double oxide. Further, it also has acontaminating phase including double oxide of silicon.

FIG. 10 is a table indicating a measurement result and a bulk density oflithium ionic conductivity in the solid electrolyte pellet of Examples 1to 5, and Comparative Examples 1 and 2.

As illustrated in FIG. 10, the lithium ionic conductivity (S/cm) of thebulk in Examples 1 to 5 is 10⁻⁴ order or 10⁻⁵ order, and is not inferioreven compared with Comparative Examples 1 and 2. On the other hand, thelithium ionic conductivity (S/cm) of the grain boundary in Examples 1 to4 is 10⁻⁴ order, and is 10⁻³ order in Example 5. In contrast, thelithium ionic conductivity (S/cm) of the grain boundary in ComparativeExamples 1 and 2 is 10⁻⁵ order or 10⁻⁶ order. Accordingly, the totallithium ionic conductivity (S/cm) of Examples 1 to 5 is 10⁻⁴ order or10⁻³ order; wherein the total lithium ionic conductivity (S/cm) ofComparative Examples 1 and 2 is 10⁻³ order to 10⁻⁶ order. That is, inExamples 1 to 5, the lithium ionic conductivity (S/cm) at a grainboundary is improved and thus the total lithium ionic conductivity ismore improved than that of Comparative Examples 1 and 2.

In Examples 1 to 5, among the doped Nb, Ta, and Sb, the metal elementssubstituted with 48 g site in the garnet-type or garnet-like typecrystal structure all have the effect of promoting the formation ofcubic crystals. It is considered that when the metal elements whichcontribute to the expression of bulk ionic conductivity and compete witheach other at the same site, a concentration gradient of the metalelement is generated between the first electrolyte 31 and the secondelectrolyte 32, which contributes to a decrease in the grain boundaryresistance in the composite of the first electrolyte and the secondelectrolyte 32. In Example 5, it is considered that a composite particleof the first electrolyte 31 and the second electrolyte 32 is dispersedby adding the third electrolyte 33, the resistance occurring at theparticle interface becomes extremely small and the total ionicconductivity is improved.

The solid electrolyte of Comparative Example 2 is doped with Si havingan atomic crystal radius of less than 78 pm, Si having an atomic crystalradius of 40 pm is not substituted with the 48 g site in the garnet-typecrystal structure but lithium is taken from the garnet-type crystalstructure so as to form a double oxide, and thus the grain boundaryresistance is increased and the ionic conductivity of the grain boundaryis decreased as compared with Comparative Example 1.

In addition, the bulk density in the solid electrolyte pellet ofComparative Examples 1 and 2 is less than 50%; whereas, the bulk densityin the solid electrolyte pellet of Examples 1 to 4 is approximately 70%to 78%. Further, the bulk density of the solid electrolyte pellet ofExample 5 with which the third electrolyte 33 is filled is approximately98%. That is, it is presumed that the first electrolyte 31 which iscrystalline of Examples 1 to 4 is more effectively used than inComparative Examples 1 and 2, and the first electrolyte 31 of Example 5is still more effectively used.

The invention is not limited to the above-described embodiments, and canbe appropriately changed within a scope not contrary to the gist or ideaof the invention which can be read from the claims and the entirespecification, and the solid electrolyte accompanying such change andthe lithium ion battery to which the solid electrolyte is applied areincluded in the technical scope of the invention.

Modification Example 1

The metal element selected from group 5 elements in period 5 or higheror group 15 elements in the period 5 or higher is not limited to two orthree kinds selected from Nb, Ta, Sb, and Bi, and four kinds thereof maybe used.

REFERENCE SIGNS LIST

1 . . . CURRENT COLLECTOR

2 . . . ACTIVE MATERIAL LAYER

3, 3X . . . SOLID ELECTROLYTE LAYER

4 . . . ELECTRODE

10 . . . LITHIUM ION BATTERY

31 . . . FIRST ELECTROLYTE

32 . . . SECOND ELECTROLYTE

33 . . . THIRD ELECTROLYTE

1. A solid electrolyte comprising: a first electrolyte which contains alithium composite metal compound; and a second electrolyte whichcontains Li and at least two kinds of metal elements selected from group5 elements in period 5 or higher or group 15 elements in period 5 orhigher.
 2. The solid electrolyte according to claim 1, wherein an atomiccrystal radius of the metal element is 78 pm or more.
 3. The solidelectrolyte according to claim 2, wherein the first electrolyte is alithium composite metal compound containing Li, La, and Zr, and whereintwo kinds of the metal elements are selected from Nb, Ta, Sb, and Bi. 4.The solid electrolyte according to claim 2, wherein the firstelectrolyte is a lithium composite metal compound containing Li, La, andZr, and wherein three kinds of the metal elements are selected from Nb,Ta, Sb, and Bi.
 5. The solid electrolyte according to claim 1, furthercomprising: a third electrolyte which is amorphous and is formed of anoxide containing Li and B.
 6. A lithium ion battery comprising: a solidelectrolyte layer formed of the solid electrolyte according to claim 1;an electrode provided on one surface of the solid electrolyte layer; anda current collector provided on the other surface of the solidelectrolyte layer.
 7. The lithium ion battery according to claim 6,wherein the electrode is formed of metallic lithium, and wherein apositive electrode active material layer containing Li is providedbetween the other surface of the solid electrolyte layer and the currentcollector.